A trooper from the Colorado State Patrol pulls down the safety tape at an entrance to the student parking lot at Arapahoe High School in Centennial, Colo., on Dec. 14, 2013. / David Zalubowski, AP

by Carolyn Pesce, USA TODAY

by Carolyn Pesce, USA TODAY

The gunman who entered a Denver area high school on Friday and shot and critically wounded a student intended to harm a large number of individuals, officials said Saturday.

Karl Pierson, 18, who died of a self-inflicted gunshot, entered the north side of the high school armed with a shotgun he bought legally, multiple rounds of ammunition strapped across his body, a machete and a backpack filled with three Molotov cocktails.

In less than two minutes he fired five shots and ignited one of the Molotov cocktails before running to the back of the school library and killing himself.

Arapahoe County Sheriff Grayson Robinson said Pierson would have shot many more people if a sheriff's deputy assigned to the school hadn't gotten to the area as quickly as he had.

"The shooter knew the deputy was in the area," Robinson said at a news briefing. "We believe that the response ... was absolutely critical to the fact that we did not have additional injury or deaths."

He added: "It is our strong belief that (Pierson) came to this school ... to utilize those multiple rounds to cause harm to a large number of individuals."

Robinson said that Pierson shot senior Claire Davis, 17, "point blank" in the face and that because everything happened so fast she did not have a chance to run. A statement he read from the family said that Davis has severe head trauma. Robinson said it appears that Pierson did not know Davis.

Robinson said that Pierson purchased the shotgun on Dec. 6 at a local retail outlet. Because he was 18 he was legally able to purchase a shotgun or long rifle in the state of Colorado.

Prior to coming to school on Friday morning, Pierson purchased multiple rounds of ammunition.

The motive for the shooting was "some type of retaliation" against a faculty member over action taken relative to the debate team, Robinson said. But Pierson was not facing any disciplinary action over the incident and was not kicked off the debate team.

Authorities spent Saturday searching a variety of properties around the metro area and conducting a large number of interviews as well as continuing to investigate the high school which has been cordoned off as a crime scene..

Tracy Murphy, a librarian and debate team coach, is believed to have been the gunman's target in the shooting.

According to KUSA-TV in Denver, Murphy implemented "active-shooter protocols" after he learned that Pierson was armed with a shotgun and asking for him on Friday. Murphy then left the scene, a move Robinson earlier said may have helped to limit the potential carnage.

An Arapahoe County deputy escorted Murphy home to pick up a few belongings on Friday night. Murphy declined to talk about the shooting. What he has to say about his interactions with Pierson likely will be a key part of the investigation.

On Saturday, students were able to return to the high school for the first time since being evacuated during the shooting Friday afternoon. They picked up their cars, but they were not allowed to go inside the school, which remains a crime scene. The school was surrounded in yellow crime scene tape.

It is unknown when students will be allowed back into the school to pick up their backpacks, cellphones or other items left behind. Classes are canceled on Monday. Finals were scheduled to start Tuesday, but those also have been canceled.

"Communication about final exams will come as we have more information, but they will not occur in any format until after the holiday break," according to a letter from the Arapahoe High School Administrative Team.

The shooting - on the eve of the first anniversary of the Newtown school massacre, in which 20 students and six staffers were killed - sent scores of terrified students and staffers at Centennial's Arapahoe High School scurrying at about 12:30 p.m. Police and other first responders quickly mobilized to surround the 2,220-student school.

The gunman also brought two Molotov cocktails inside the school and exploded one, KUSA-TV reported. The other was found and removed by the bomb squad.

The incident unfolded when the armed student entered the west side of the school from a student parking lot. He told other students he was interested in confronting a specific teacher.

Many students locked themselves in classrooms until first responders arrived. Some said they heard several gunshots in a hallway near the school library.

"We were shaking, we were crying, we were freaking out,'' 9th grader Whitney Riley told CNN.

Jessica Girard was in math class when she said she heard three shots.

"Then there was a bunch of yelling, and then I think one of the people who had been shot was yelling in the hallway, 'Make it stop,' " she said.